Browser loses session sometimes

I have created a rather intensive asp.net application that has about 10 or so frames in the browser. One of these frames refreshes every 30 seconds and has a large number of database queries.

Now this page also keeps the user session alive.

This asp.net application is served on our intranet, and on some of the client browsers, the page that refreshes every 30 seconds sometimes just goes blank, and then the user loses their session information, and have to log back in again.

Why would this happen on some browsers and not others?

How does one fix something like this?

There is nothing in the event log about the asp.net wp restarting or any IIS errors...